In a total knee arthroplasty, portions of the distal femur, and proximal tibia are replaced by prosthetic components made of metal alloys, high-grade plastics and polymeric materials. Much of the other anatomical structure of the knee, such as the connecting ligaments, remain intact.
The human knee is a very complex joint because the surfaces must roll and glide properly as the knee alternates from flexion to extension. Prostheses attempt to conform to the complexity of the joint, and attempt to replicate the more complicated motions and to take advantage of the posterior cruciate ligament (PCL) and collateral ligaments for support.
Up to three bone surfaces may be replaced during a total knee arthroplasty: the distal portion of the femur, encompassing the medial and lateral condyles, the proximal portion of the tibia, and occasionally, the posterior surface of the patella. Components are usually designed so that metal articulates against plastic, which provides smooth movement and results in minimal wear.
The metal femoral component curves around the distal end of the femur and has an anterior indentation so the patella can articulate smoothly as the knee alternates between flexion and extension. Usually, one large femoral component is applied the distal end of the femur. If only one condylar portion of the femur is damaged, a smaller component may be used. This is referred to as a unicompartmental knee replacement. Some designs such as posterior stabilized designs, have an internal structure that cooperates with corresponding structure on a tibial component to help prevent the femur from sliding anteriorly too far on the tibia when the knee is placed in flexion. The tibial component is typically a generally flat metal platform with a polyethylene bearing. The bearing may be part of the platform or separate with either a flat surface or a raised, sloping surface. The patellar component is typically a dome-shaped piece of polyethylene that duplicates the shape of the patella anchored with bone cement.
In a conventional total knee arthroplasty procedure, a patient's knee is placed in flexion so that all surfaces to be replaced are patent and accessible to a surgical team. A standard surgical approach is through a sagittal incision on an anterior surface of the knee slightly medial to the patella, although some surgeons will approach the joint from an incision lateral and superior to the patella. The incision through the skin is usually 6″ to 12″ in length. The large quadriceps muscle and the patella are moved to the side to expose the bone surfaces of the knee.
After taking several measurements to ensure that a new prosthetic component will fit properly, the surgeon begins to resect portions of the distal femur and/or proximal tibia. Depending on the type of implant used, the surgeon may begin with either the femur or the tibia. Special instrumentation such as cutting blocks can be used to ensure accurate resection of the damaged surfaces at the distal portion of the femur. The devices help shape the distal end of the femur so it conforms to the inside surface of the new prosthesis. If it is necessary to remove portions of the condyle or other distal portions of the femur, the surgeon typically uses instrumentation which is connected to the femur in order to resect the necessary portions of the femur so that the implant can be properly positioned or oriented. In some cases however, such as a revision case, the distal portion of the femur is so severely deteriorated that it requires augmentation before the implant can be installed.
The tibia is then modified by making a transverse cut across the bone and a central portion of the tibia is prepared. The surgeon removes just enough of the tibia so that when the prosthesis is inserted, it recreates the joint line at the same level as prior to surgery. If any ligaments around the knee have contracted due to degenerative disease or injury before the surgery, the surgeon carefully releases them so that they function as close to the normal state as possible.
During the total knee arthroplasty, proper positioning of the superior/inferior joint line between the femoral component and tibial component is critical to a successful operation. Joint line malpositioning can adversely affect the patellafemoral mechanics and may lead to anterior knee pain and may reduce range of motion. Proper superior/inferior joint line positioning is equally critical in a revision total knee arthroplasty. During a revision total knee arthroplasty however, determining the superior/inferior joint line position is particularly difficult because there is often significant deterioration of the proximal fibia and distal femur and, therefore, an absence of adequate anatomical landmarks for an accurate joint line positioning. To aid in the determination of the proper superior/inferior position of the joint line between the femoral component and tibial component, a trial femoral component is often used. During the replacement procedure, testing of proper positioning for the prosthetic components can be conducted with the trial components in place without exposing the actual components to potential wear or degradation.
Existing methods of determining proper superior/inferior joint line positioning include ratios that are determined based upon the position of the tibial plateau relative to the length of the patella tendon. Ratios dependent on the tibial plateau position may be faulty because they must assume the correct level of the tibial plateau which may not be achieved. Ratios dependent on the length of the patella tendon can be time-consuming, confusing and do not have a relationship to the total knee arthroplasty instrumentation. A need exists, therefore, for a joint line positioning apparatus that will help determine the proper superior/inferior joint line position between the femoral component and the tibial component during a total knee arthroplasty or a revision total knee arthroplasty when existing anatomical landmarks or portions of the femur are damaged or nonexistent.